I can't speak for Forza because I've only played casual split screen on friends' consoles, but I think it's been significantly upped from GT5.
Along with the buy-and-install mods like turbos/weight reduction, you can manually fine tune more things. For example, you can manually adjust any aspect of your suspension, like spring stiffness, camber, and dampening. You can add a chosen amount of weight/ballast to a chosen position in your car. You can adjust all the numbers/ratios in your tranny, LSD, and axle-power distributor. Just to name a few.
My one gripe is the lack of exterior customization; Forza has way cooler points there, like being able to create custom wraps, whereas in GT6 it's pretty much limited to a plain solid color and aftermarket wheels.